More particularly, the invention proposes a protection device for a chopping supply of the type comprising a voltage converter whose secondary is connected to an electrical load and whose primary is connected to an electrical source such as a light source of the xenon lamp type (also referred to as a discharge lamp) and controlled by a power switch.
Such a chopping supply is intended, for example, to supply any lighting and/or signalling device, of the motor vehicle headlight type. The secondary of the voltage converter is connected to the headlight and the primary of the voltage converter is connected to a battery of the motor vehicle on-board system.
In a chopping supply, the phenomena of thermal runaway of the converter, due to high temperatures, appear and affect the characteristics of the electronic components.
Such a thermal runaway occurs particularly in an automobile environment, the temperature being high in the engine compartment where the headlight and its control supply proper are situated.
Likewise, in production, the burn-in of the controlled supplies of the headlights is effected partly on the lamp. After numerous burn-in cycles, the lamps are worn and no longer remain lit. The controlled supply or ballast is then constantly in lighting phase with overpower regimes, which, added together at a high temperature, give rise to thermal runaway of the voltage converter and therefore destruction of the power switch which effects the chopping of the voltage supply to the primary.
The phenomena of thermal runaway thus result in a high current surge dangerous to the primary of the voltage converter, especially when it is produced in the form of a magnetic transformer with coils. A critical current level causes destruction of the power switch. A problem is therefore posed of detecting a critical current level and acting rapidly before the destruction of the components.
In the document EP-A2-1.043.919, a protection device is described which monitors the current in the power switch, particularly a transistor, by measuring the voltage drop at its terminals, particularly the voltage between the drain and source of the transistor, proportional to the current, in the conductive state of the transistor. A comparator opens the transistor when the voltage drop is too great.
One drawback of such a solution is that the measurement of the voltage is dependant on the dynamic resistance of the transistor, whose tolerance is not very precise, and consequently the protection is not very reliable.
Another drawback is that, when the ambient temperature is high, the converter is more sensitive to thermal runaway which causes damage to the transistor, or even destruction of the transistor. However, the solution proposed by the document does not take the temperature into account. The transistor is cut off only at high currents flowing in the primary induction coil. The solution of the state of the art does not make it possible to anticipate the appearance of such currents.
Another drawback is that, when the current is very high, phenomena of magnetic saturation of the transformer used as a voltage converter appear. The power delivered by the secondary is no longer sufficient to supply the load connected to the secondary, particularly a headlight installation. The control unit for the headlight installation sends a power demand signal to a unit controlling the chopping supply. The induction current of the voltage converter increases but, the transformer still being saturated, the chopping supply cannot always supply the power necessary for supplying the headlight installation. However, the regulations require consistency and an optical flux level at the headlights. The saturation of the transformer is thus a significant problem for the supply to the headlights. In addition, the maintenance of a high power demand by the control unit for the headlight installation also maintains the situation causing a dangerous current surge, which increases the problem of destruction of the switch all the more.